1. Field of the Invention
The present invention relates, generally, to a method and system for detecting the early stages involved with the jamming of a core sampling device, before catastrophic jamming has occurred, and more particularly, to a method and system for taking remedial action, based upon such early detection, to prevent such catastrophic jamming of the device.
2. Description of the Background
Conventional devices for obtaining core samples from an earth borehole comprise a tubular housing attached on one end to a special bit commonly known as a core head, and at the other end to a drill string extending to the earth's surface. The tubular housing in turn comprises an inner and an outer barrel with a space between. During normal drilling the drilling fluid flows through the interior of the inner barrel. When a core sample is required, the flow passageway in the inner barrel is blocked, often by dropping a ball from the earth's surface, thus diverting the flow into the space between the inner and outer barrel and down through the bit. The absence of flow in the inner barrel allows the earth formation to enter and fill the barrel, which is then subsequently recovered as a core.
One disadvantage of conventional methods is that in many situations jamming of the core in the inner barrel can occur. This causes the drill string weight to be transferred substantially away from the outer barrel to the core and to the inner barrel, causing fracture of the core and cessation of core acquisition. In this event either penetration of the formation ceases or drilling does in fact continue, resulting in milling of the formation by the corehead, rendering that portion of the formation permanently unrecoverable since the milled material simply gets pumped to the surface with other rock cuttings. This situation is particularly undesirable in formations where coring as a means of evaluation is imperative due to the inherent limitations of wireline logging in those types of formations. This is discussed in the paper by Bradburn and Cheatham entitled "Improved Core Recovery in Laminated Sand and Shale Sequences", Journal of Petroleum Technology, December 1988.
The prior art, in addressing the problem of jamming, has had only limited success. For example, in U.S. Pat. No. 4,492,275 to Frank L. Campbell et al., a core jamming indication is sought by noting rotation of the inner barrel which is normally stationary relative to the outer barrel. However, by the time such rotation has occurred, the core is almost certainly broken off. Furthermore, the method relies on use of expensive Measurement While Drilling (MWD) equipment to signal this event to the surface.
This type of system will, in most cases, result in catastrophic jamming of the core sampling device, or in one of the other equally troublesome problems above discussed, before the surface personnel are aware of the problem.
As another example of the prior art, in PCT Application No. WO92/02707 filed by Arno et al., a downhole core sampling is disclosed in which jamming of a core causes the inner barrel to be pressed against the outer barrel, cutting off the flow passage between the barrels, thus causing a significant increase in the pressure of the drilling fluid detectable at the earth's surface. However, the mechanical means required to produce the flow passage blockage is triggered by significant loading on the core, at which point core fracture has in all probability occurred.
Thus, each of these two prior art attempts to detect jamming involve the same problems, that of (1) requiring modification of the core sampling devices themselves, and (2) resulting all too often in catastrophic jamming of the device or fracturing of the core sample.
It is therefore the primary object of the present invention to provide a new and improved method and system for the early detection of the jamming of the core sampling device, before major problems develop, thus enabling timely remedial action to be taken.
It is another object of the invention to provide a new and improved method and system for determining the movement, or other activation of parts in downhole tools.